Fan speed detecting circuit

Abstract

A fan speed detecting circuit includes a resistor installed in the grounding line of the power circuit of the DC fan motor of a cooling fan, a capacitor, a diode connected between the resistor and the capacitor, and a comparator having an inverter terminal connected to the output end of the diode through the capacitor and a non-inverter terminal connected in parallel to the input end of the diode, wherein a transient pulse current is produced at the resistor upon operation of the DC fan motor, causing the comparator to output a pulse frequency wave indicative of the revolving speed of the DC fan motor.

What the invention claimed is: 
     
       1. A fan speed detecting circuit installed in the power circuit of a DC fan motor to detect the revolving speed of the fan motor, the fan speed detecting circuit comprising: a resistor installed in the grounding line of the power circuit of the DC fan motor;   a diode having an input end connected to said resistor and an output end;   a capacitor;   a comparator having an inverter terminal connected to the output end of said diode through said capacitor and a non-inverter terminal connected in parallel to the input end of said diode;   wherein a transient pulse current is transmitted from the DC fan motor to said resistor upon operation of the DC fan motor, and turned into a pulse voltage by said resistor, causing said comparator to output a pulse frequency wave indicative of the revolving speed of the DC fan motor.   
     
     
       2. The fan speed detecting circuit of claim 1 wherein the impedance value of said resistor is below 0.5 ohm.